This invention relates to hot melt inks and, more particularly, to a new and improved transparent hot melt ink for use in ink jet systems.
In order to make excellent color reproductions on paper or vivid overhead projection transparencies, it is necessary to use highly transparent colored inks. These are normally supplied in a four-color process set, i.e., cyan, magenta, yellow and black. Conventional color printing often utilizes oil-based inks which normally incorporate a polymeric, film-forming, highly transparent vehicle to carry colorant to the substrate to be printed. Those polymeric vehicles, however, tend not to be suitable for inks used in ink jet systems because they have very high melt viscosities.
Conversely, low melt viscosity, sharp melting wax-like materials such as stearic acid, stearone and laurone, behenic acid, tristearin, terephthal alcohol, hydrogenated castor oil, propylene glycol monohydroxystearate, n-stearyl 12 hydroxystearamide etc., while satisfying the melt viscosity requirements of ink jet systems, form highly crystalline opaque solids on cooling. Similarly, most waxes, such as polyethylene, carnauba, candelilla, microcrystalline, and oxidized or modified polythene waxes, also tend to crystallize on cooling. Because of their crystalline content, such inks, even when mixed with transparent constituents, are not completely transparent and, when jet printed, do not project vivid colors in overhead projection transparencies, for example.
Heretofore, hot melt ink formulations have been proposed which comprise blends of natural waxes (e.g., U.S. Pat. No. 4,484,948), C.sub.18 -C.sub.24 fatty acids or alcohols (European Published Applications No. 0 181 198 and 0 099 682) or esters didodecyl sebacate (U.S. Pat. No. 3,653,932), and other synthetic blends of C.sub.20-24 acids and C.sub.14-19 acids or alcohols and ketones (U.S. Pat. No. 4,659,383). U.S. Pat. No. 4,741,930 discloses a method for color printing by the super-position of semitransparent, solid, optically clear hot melt inks and suggests fatty acids, such as stearic acid and oleic acid, polyethylene, and blends or mixtures of such materials, as base materials for the vehicle.
U.S. Pat. No. 4,684,956 discloses a thermoplastic ink composition comprising blends of synthetic microcrystalline wax and hydrocarbon resin having a viscosity of 3-20 cps at the operating temperature to achieve low viscosity coupled with good adhesion.
None of the prior art, however, provides a hot melt ink containing a nontransparent constituent which is substantially transparent while retaining all of the other properties required for ink jet printing.